Machines with high precision positioning mechanisms are used in many industries for accurately measuring the tolerance of machine parts and other components as well as for positioning tools for performing highly precise operations. Typically, either a tool or a touch sensitive probe for position measurement is mounted on the end of a support structure that allows the probe or tool to be moved in three dimensions by selective translation along three orthogonal axes.
Gantry type support structures are commonly used in such machines, particularly coordinate measuring machines. Such a gantry-type structure typically includes a base, a table resting on the base, a gantry structure which rides on parallel, spaced rails, generally referred to as X-rails, which are supported by the base, and a carriage which rides on the gantry structure. A vertically movable element, typically referred to as a Z-rail, includes a touch sensitive probe or tool disposed on the lower end thereof. The Z-rail moves vertically with respect to the carriage, while the carriage travels horizontally along a rail, usually referred to as the Y-rail, disposed on the gantry structure. Air bearings are used to facilitate movement of the gantry structure on its associated rails, as well as movement of the carriage and Z rail. Typically, the gantry structure is driven only along one rail. The probe or tool disposed on the end of the Z rail therefore can be moved in three dimensions to be positioned at any point on the table.
Electronic sensors are provided on each rail for sensing the position of the probe or tool in terms of its X, Y and Z coordinates. Typically, a microcomputer is provided within the device which causes the probe to be moved through a sequence of specified X, Y and Z locations and remaining at each for a specified period of time to either measure or work on the part resting on the table.
In use, the gantry type structure is rapidly accelerated and decelerated to deliver the probe or tool to the point at which it is to perform its work or measurement. Such rapid movements are necessary so that measurements or work at each point can be performed rapidly and the total time for testing a part or performing work is not intolerably long. Such rapid accelerations and decelerations produce vibrations in the machine.
One source of such vibrations is the gantry structure itself. In part because such gantry type structures are generally driven only along one rail, vibrations are caused by the structure's inherent inertia. A torque is produced about the point at which the drive is coupled to the gantry-type structure during both acceleration and deceleration, and it may be assumed for most purposes that the torque is applied at the center of gravity of the gantry-type structure and has a moment arm which extends from the point of coupling of the drive to the structure to the center of gravity thereof. Since the gantry-type structure has a certain elasticity, and since during deceleration continued movement of the structure is resisted by the drive and by the bearings which maintain the proper positioning of the structure, this torque causes vibrations to be set up in the gantry about the point at which the gantry is coupled to the drive. These vibrations are then transferred to the air bearings coupling the gantry to the X-rails, and these air bearings can act like a spring.
Another source of vibrations is the Z-rail which typically is in an extended position depending downwardly from the carriage during movement thereof. Rapid acceleration and deceleration produces oscillation of the distal end of the Z-rail about the carriage, or about the point at which it is coupled to a Z-rail drive. These vibrations are created in much the same way as are the vibrations in the gantry structure, since the Z-rail also has a certain elasticity and since the inertia of the Z rail applies a torque about the point at which the Z-rail is coupled to the air bearings in the carriage. The Z-rail vibrations contribute significantly to the overall vibrations in the machine.
Finally, vibrations are also present in the table upon which the part to be tested or worked on is placed. These table vibrations are produced by the torque applied to the rails by the X-rail drive during rapid accelerations and decelerations of the gantry, and tend to be aligned in the direction of acceleration or deceleration. Such table vibrations can produce either movement of the entire table back and forth in a direction parallel to the X-rails, or twisting of the table about a vertical axis in which the one side of the table associated with one of the rails is moving in one direction, somewhat parallel to that rail, while the other side of the table associated with the other X rail is moving in an opposite direction somewhat parallel to its associated rail.
The vibrations produced in the carriage by its rapid acceleration and deceleration are relatively small, and do not contribute significantly to the overall vibrations in the machine. The carriage is positioned close to the Y-rail, and is held in position by a number of opposed air bearings. Because no portion of the carriage extends outwardly away from the Y-rail, little or no torque is applied by the carriage itself during acceleration, or deceleration.
Obviously, it is desirable to damp the vibrations out of the system as quickly as possible to avoid errors. For high precision coordinate measuring machines, it is essential that the measurements be taken when no vibrations are present, so that the precise alignment can be maintained. It is therefore necessary to delay making a measurement after rapid movement of the probe until the vibrations are reduced to an acceptably low level. The time required for the vibrations in the machine to damp out of the system is termed the settling time.
Certain characteristics of a gantry type positioning mechanism have a substantial effect on the amplitude and frequency of the vibrations in the structure. Very soft or dynamically marginal air bearings tend to increase the amplitude of vibration and settling time of the system. Also, if the bearings are unstable, particularly at a resonant frequency of the gantry type structure, the settling time will be increased. The amplitude of the vibrations and the length of the settling time also depend on the stiffness of the gantry structure and the Z-rail, as well as on the distance of the center of mass of the gantry from the X-axis drive and on the distance of the center of mass of the Z-rail from its drive. The amplitude of the vibrations will increase as the carriage is moved along the Y-rail away from the X-axis drive or as the Z-rail is moved away from the X-axis drive or as the Z-rail is extended from the carriage. This increase in amplitude occurs because the moment arm of the force and thus the applied torque increases.
Therefore, it is an object of the present invention to reduce vibrations in a gantry-type machine having a high precision positioning mechanism.
It is another object of the present invention to reduce vibrations in a coordinate measuring machine.
It is a further object of the present invention to provide vibration dampers for a coordinate measuring machine.
It is another further object of the present invention to reduce the settling time of vibrations in a gantry-type structure which are induced by acceleration and deceleration of the machine components during rapid measurements.
It is yet another object of the present invention to improve the throughput of a coordinate measuring machine.